You should check what period you have set up for "Discard items older than" select at https://beams.web.cern.ch/admin/config/services/aggregator/settings .Usually it is set up for around one month and as I see all the entries you have in that feed are from 2015 year or older, so the right option for "Discard items older than" would be "never", so the old items won't be discarded before display. But you have to keep in mind that changing this option will affect ALL feeds that you have on your website and this may end with displaying a lot of content from the feeds, and make the website very crouded.